1) Ivan thinks that people need more structure and less freedom. He thinks that the church should be held responsible for the sins of all men.
2) The GI believes that Jesus misunderstood mankind. He thought that they would be better off given many freedoms. The GI disagrees because he believes that man needs more structure and less choice.
3) The Grand Inquisitor suggests that the church can relieve the human's "burden of freedom" by giving them structure and taking their sins upon themselves.
4) When people have the freedom to make their own choices, when the wrong choice is made, they are the ones held accountable. When people just follow the structured rules there is no one to blame but the people giving the orders.
It seems so dumb that the Grand Inquisitor would rather aimlessly lead people then let them make their own mistakes, at least thats what I think.